It is not as yet possible to describe the molecular basis of the well-known increase in the yellow color and visible, long-wavelength fluorescence which occurs with aging of the human lens and with the appearance and progress of certain types of senile cataract. An analytical comparative study of the absorbing/fluorescent properties of single normal and cataractous human lenses has been undertaken in order (1) to distinguish between modifications that are related to the age of the lens as distinct to those related to cataractogenesis; (2) to determine the source of the apparent heterogeneity of chromophores and fluorophores seen in pools of cataract lenses; (3) to define which chromophores/fluorophores of the cataract lenses (more readily available than normal lenses) are quantitatively more important to attempt the identification of their chemical structure; and (4) to "a posteriori" try to define which of the lens crystallins has been most modified during the aging and cataractogenic processes. When single lenses are submitted to extensive proteolytic digestion a number of polypeptides that have resisted digestion can be isolated through gel filtration HPLC (high-performance liquid chromatography) of the digest. The qualitative chromotographic profiles are similar from lens to lens but the size distribution of the peptides clearly depends on the characteristics of the lens. These peptides represent the most modified regions of lens proteins. From them we expect to obtain fragments carrying chromophores/fluorophores that can be submitted to conventional methods of molecular structure determination.